1. Field of the Invention
The present invention relates to a method for producing a molded article of a ceramic oxide superconductor.
2. Description of Related Art
A superconductor exhibits complete diamagnetism under superconductive conditions and has no potential difference therein in spite of flow of stationary current therein. By using this characteristic of the superconductor, many applications of superconductors as mediums for electric current have been proposed.
Application fields of superconductors include versatile technical fields such as an electric power field (e.g. MHD power generation, power transmission, power storage, etc.), a motive power field (e.g. linear motor (magnetic letivation) trains, electromagnetic propulsion ships, etc.), and a measuring field in which the superconductor is used as a highly sensitive sensor for a magnetic field, a microwave, radiation and the like (e.g. NMR, therapy using .pi. -meson, high energy physical experiment apparatus, etc.).
In addition, in the electronic components including a Josephson element, the superconductor is expected to provide an element which can not only decrease consumed power but also work at a very high speed.
The superconductive phenomenon has been found in specific metals or organic compounds at extremely low temperatures. Namely, among classical superconductors, Nb.sub.3 Ge is said to have the highest critical temperature Tc for superconductivity of 23.2.degree. K and this temperature has been considered as the upper limit of Tc for a long time.
Hitherto, to realize the superconductive phenomenon, a superconductor should be cooled to a temperature lower than Tc with liquid helium which has a boiling point of 4.2.degree. K However, the use of liquid helium greatly increases technical burden and cost because of necessity of a cooling system including an apparatus for liquefying helium, which prevents practical application of the superconductor.
Recently, it was reported that a sintered material comprising oxides of elements of IIA or IIIB group of the periodic table can act as a superconductor at a high critical temperature and is expected to accelerate practical application of superconductor technology. From already available reports, compound oxides having a crystal structure similar to the perovskite crystal structure such as (La,Ba).sub.2 CuO.sub.4 or (La,Sr).sub.2 CuO.sub.4 are known as superconductors having high Tc. These compound oxides have a Tc of 30 to 50.degree. K, which is far higher than that of the classical superconductors. In addition, a Ba-Y-Cu type superconductor is reported to have Tc higher than the liquid nitrogen temperature.
Hitherto, the ceramic oxide superconductor is processed into a molded article by substantially the same method as adopted for molding non-superconductive oxide ceramic oxides. For example, powdery raw material oxides are pressed and then sintered to produce a molded article. If necessary, the pressed powder is presintered before sintering.
The ceramic oxide superconductor comprising a La-Ba-Cu base oxide, La-Sr-Cu base oxide, Ba-Y-Cu base oxide or the like has been developed. When a molded article of such ceramic oxide superconductor is produced from a powdery raw material by the conventional method as described above, it is difficult to provide a crystalline structure of the ceramic oxide superconductor and to increase a size of crystal grain boundary, so that the produced article of ceramic oxide superconductor has a relatively small critical current density.
In addition, since the molded article of ceramic oxide superconductor produced by sintering has a smaller degree of shaping freedom, it is difficult to produce a wire from the ceramic oxide superconductor. In the production of a wire, the ceramic oxide superconductor is processed in the form of a thin wire having a diameter of 1 mm or less and extended to a length of several meters. Further, the wire cross section should have a suitable shape such as a pipe, etc.
For producing a molded article, there are known several methods including a method comprising melting a sintered ceramic oxide superconductor and molding the superconductor melt to form an article having a desired shape, and a method comprising covering the sintered ceramic oxide superconductor with a metal sheath to stabilize the superconductor and then wiring the sheathed superconductor.
In such conventional methods, the once sintered ceramic oxide superconductor is melt or thermally treated at high temperature for a long period of time. Therefore, the superconductive phase created in the ceramic oxide superconductor tends to be modified or lost.
It is proposed to anneal the wire of sintered ceramic oxide superconductor at a temperature of about 950.degree. C. to recreate the superconductive phase in the wire. However, the molded article tends to be broken during annealing. Further, since some of the elements such as copper and oxide are dissipated during annealing, the superconductor cannot keep its homogeneous composition.
Because of the above problems found in the conventional processing methods, it seems to be hardly possible to stably process the ceramic oxide superconductor and to produce a molded article of ceramic oxide superconductor having a desired shape.